This invention has to do with non-destructive testing of high efficiency particulate air filters. More, particularly, the invention is concerned with novel testing procedures for HEPA filters involving an advantageous aerosol of solid particles which are hydrophobic and non-toxic.
HEPA is an acronym for high efficiency particulate air filters. Such filters comprise a rigid frame and a filter pack, sealed in the frame; the filter pack comprising a tightly packed axially extended alternating series of separators and parallel reverse turns of glass cloth. The art of HEPA filters has been highly developed and numerous of these particularly effective filters are used in hospitals, clean rooms, electronic assembly and environmental safeguard installations for the removal of extremely small contaminants from an airstream, e.g. bacteria and other materials down to the size range of 0.2 micron. Because of the numerous critical applications of these filters, extensive testing is done upon them at points during manufacture, and during and after installation as well. For such purposes, there have been developed testing techniques involving the impinging of an aerosol against the filter and the recording on the opposite side of the filter of passage of the test material through the filter.
The present standard testing material in the industry is dioctyl (or dinonyl) phthalate. This material generates a fine aerosol of suitably sized particles which effectively seek our failure points in a filter or a filter installation. DOP based test readings are not entirely free from error however, and such errors can become cumulative. More particularly, DOP has been found to coat the optics in all optical instruments and scientific apparatus in the room necessitating periodic restandardization. Other shortcomings of DOP as a test material include a tendency to pick up ambient moisture, adversely affecting long term stability, potential flammability, and the ability to wick along glass cloth, because of its liquid nature, possibly causing erroneous readings.